The present invention relates generally to stimulated Raman scattering (SRS) lasers, and more specifically to an SRS laser which uses a third enhancing beam to enhance the pump energy-to-Stokes energy conversion.
Laser action was first established in a optical cavity which allowed photons to be reflected back and forth so as to build up the intensity of the radiation. A technique for producing intense tunable coherent radiation is stimulated Raman scattering. In employing SRS, an intense laser beam is converted into a beam of another frequency by coherent stimulation of a Raman scattering processes. The SRS laser phenomena works as described below.
Light scattering by various substances involves the scattering of a light beam which traverses a transparent dielectric sample (solid, liquid or gas), the scattering being caused by the different dielectric characteristics of the sample. This linear or thermal scattering of light has been observed without lasers for a long time. With the advent of the laser and its high intensity, collimation, and monochromaticity, research in light scattering spectroscopy has been greatly facilitated.
SRS concerns scattering from the optic vibrational modes and is achieved by directing a pump beam from a laser, usually pulsed, through a Raman medium. The pump beam decomposes into a Stokes wavelength output, which is photon-like, and a Raman wavelength output.
Normal stimulated Raman scattering is done in a gas cell with pump and Stokes seed input beams. The pump photons excite the gas molecules from their ground state and the Stokes seed photons stimulate the de-excitation of the molecules back to a lower state (emitting more Stokes photons in the process). The task of enhancing the Stokes output is alleviated to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated by reference.
U.S. Pat. No. 3,417,346 issued to Yatsiv; PA1 U.S. Pat. No. 4,048,516 issued to Ammann; PA1 U.S. Pat. No. 4,264,877 issued to Grischkowsky et al; PA1 U.S. Pat. No. 4,315,224 issued to Ezekiel et al; and PA1 U.S. Pat. No. 4,318,057 issued to Buchwald et al.
The references cited above describe prior art stimulated Raman scattering laser systems. Grischkowsky et al disclose a laser which uses various plural pumping arrangements and discusses both a two photon process and a three-photon process. Stimulated Raman scattering is also disclosed in the Ammann patent.
Yatsiv is concerned with a laser having Raman material for double quantum absorption and Buchwald et al speak of a Raman laser which uses two optical pumping mechanisms. Ezekiel et al use three pumping lasers on a molecular beam, with the second and third pumps at a point further down the beam from the first optical pumping source. Also, in the Ezekiel et al construction two of the optical pumping beams are at the same frequency.
In view of the foregoing discussion, it is apparent that stimulated Raman scattering systems represent an emerging technology in which there remains a need to enhance the Stokes radiation output. The present invention is intended to satisfy that need.